Circuit that automatically switches between supplying a microprocessor with a first voltage and a second voltage

ABSTRACT

A circuit that automatically switches the power supply voltage PVDD provided to a CPU between 3.3 volts and 5 volts. The circuit detects whether the CPU installed in a socket is a 3.3-volt part or a 5-volt part by determining the state of a voltage detect sense pin provided by the socket. If the voltage detect sense pin is driven low, that indicates a 3.3-volt CPU is being used. If a 5-volt CPU is installed, the voltage detect sense pin is left floating by the CPU, which allows a pullup resistor to pull the voltage detect sense pin high. The power supply voltage provided to the CPU is regulated through a power field effect transistor (FET). The gate of the power FET is connected to the output of a voltage reference source and is coupled to a 12-volt supply signal. If the voltage detect sense pin is pulled high, the voltage reference source is turned off, allowing the 12-volt supply signal to drive the gate of the power FET. This in turn allows the power FET to pass a 5-volt supply signal to the CPU supply signal PVDD. If the voltage detect sense pin is pulled low, the voltage reference source is turned on to drive the gate of the power FET to approximately one threshold voltage above 3.3 volts. In response, the power FET passes only 3.3 volts to the CPU supply signal PVDD.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to controlling a power supply voltage provided toa microprocessor, and more particularly, to a circuit that automaticallyswitches between supplying the microprocessor with a first voltage or asecond voltage.

2. Description of the Related Art

Over the past decade, the performance and speed of microprocessors haveimproved dramatically. The number of transistors that can be fitted ontoa microprocessor chip number is in the millions. Such densities areallowed primarily with the use of complementary metal oxide silicon(CMOS) technology, which allows for significantly lower powerconsumption over prior bipolar and NMOS technologies. However, even withthe use of CMOS technology, power dissipation by today's highperformance microprocessors is a significant problem. To reduce powerconsumption, microprocessors such as the 486SL and Pentium® CPUs fromIntel Corporation include built-in power management functions. Theenergy efficient CPUs include a stop clock and auto halt mode. In stopclock mode, the clock speed of the microprocessor can be reduced or evenstopped altogether, which reduces the power dissipation of themicroprocessor when it is idle. The CPU enters into the auto halt modeafter execution of a halt instruction. This allows the microprocessor tobe placed into low power mode via software.

Neither of the above features, however, address how the powerconsumption of a microprocessor can be reduced while it is active. Onepopular method of reducing power consumption while the microprocessor isrunning is to reduce its power supply voltage from the industry standard5 volts down to 3.3 volts. Lower voltages have also proved to benecessary as die sizes continue to shrink. The small dimensions thatexist on a chip require a lower operating voltage to avoid problemsassociated with high current densities. With the trend towards 3.3 voltmicroprocessors chip manufacturers have had to provide bothmicroprocessors that operate at 5 volts and 3.3 volts.

This presents the computer manufacturers with a problem, however, as thecomputer system must be capable of providing two different power supplyvoltages to the microprocessor. One solution is to include a 5-voltpower supply voltage on all computer systems, adding a conventionalregulator for providing a 3.3 volt power supply voltage in the 3.3-voltcomputer systems. However, this requires that the manufacturing processbe different for 3.3-volt and 5-volt computer systems. Single-partsolutions exist where the same part can be used with both 3.3-volt and5-volt microprocessors. However, these parts include linear regulators,which are relatively expensive components. Thus the use of the separate,conventional linear regulator is undesirable, as it adds cost to themanufacture of the computer systems for several reasons.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a low-cost powersupply control circuit that can be used with microprocessors thatoperate at either a first or second supply voltage. The power supplycontrol circuit according to the present invention includes regulatorcircuitry that selectively provides a microprocessor supply voltage at afirst voltage or a second voltage, those voltages preferably being 3.3volts and 5 volts, respectively. The computer system preferably includesa socket for receiving a microprocessor, and the socket includes a pinreferred to as the voltage detect sense pin. Microprocessors operatingat 3.3 volts drive the voltage detect sense pin low, whereas 5-voltmicroprocessors leave the voltage detect sense pin floating, allowing apullup resistor to pull the pin high. The voltage detect sense pin fromthe microprocessor socket is connected to the regulator circuitry in thepower supply control circuit. The regulator circuitry includes a powerfield effect transistor (FET), whose drain is preferably connected to a5-volt source, and whose source provides the power supply voltage to themicroprocessor. The gate of the power FET 200 in the preferredembodiment is connected to a voltage reference source, which drives thegate of the power FET to approximately 12 volts if the voltage detectsense pin is pulled high by the pullup resistor, and which drives thegate to one threshold voltage of the power FET above 3.3 volts if thesense pin is driven low. Thus, if the voltage detect sense pin is pulledlow, the power FET drives the microprocessor supply voltage to 3.3volts. Otherwise, if the voltage detect sense pin is pulled high, thepower FET is completely turned on and allows the 5-volt voltage to passto the microprocessor supply voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of 5 the preferred embodiment isconsidered in conjunction with the following drawings, in which:

FIG. 1 is an exemplary computer system incorporating the power supplycontrol circuit according to the present invention; and

FIG. 2 is a schematic diagram of portions of the power supply controlcircuit of Figure I that regulates the voltage provided to amicroprocessor in the computer system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, an exemplary computer system C incorporating apower supply control circuit according to the present invention isshown. The computer system C includes a socket 100 for receiving a hostCPU or processor 101, which is conventionally a microprocessor such as a486 or Pentium® processor from Intel Corporation. It is contemplatedthat instead of using the socket 100, the CPU 101 can be directlysoldered to the motherboard. The type of microprocessors that can beused with the computer system C include the 486SX, 486DX, 486DX2, 487SX,486DX4, and Pentium® P24T processors. In addition, microprocessors fromother manufacturers compatible with the 486 processor can also be used.The 486SX, 486DX, 486DX2, and P24T processors operate at 5 volts, whilethe 486DX4 operates at 3.3 volts. Certain of the microprocessors fromthe other manufacturers also operate at 3.3 volts. The socket 100receives a pin or plurality of pins PVDD, which is the microprocessorpower supply pin or pins driven to 5 volts or 3.3 volts, depending onwhich microprocessor is used. The socket 100 also includes a voltagedetect sense pin VOLDET. When a 3.3-volt microprocessor is inserted intothe socket 100, the microprocessor drives the pin VOLDET low. If a5-volt microprocessor is inserted, the pin VOLDET is left floating.

A host bus 102 is connected to the CPU 101 to act as a first bus in thecomputer system C. A main memory system 104 is also connected to thehost bus 102 to act as the main memory of the computer system C. A videosystem 106 is further connected to the host bus 102 to allow for a highperformance video system.

A controller 110 is connected between the host bus 102 and an ISA orIndustry Standard Architecture bus 112. The controller 110 includes abus controller portion, a memory controller portion and a systemperipheral control portion. The system peripheral control portionincludes certain common peripheral devices used in the computer system Csuch as timers, an interrupt controller, and a DMA controller. The buscontroller portion of the controller 110 controls bus cycles on the hostbus 102 and the ISA bus 112. The memory controller portion providesappropriate signals to the memory 104, which preferably is made up ofdynamic random access memories (DRAMs). A number of ISA slots 116 forreceiving interchangeable circuit cards are present on the ISA bus 112.

Appropriate buffer and transceiver logic 120 is connected between theISA bus 112 and an X bus 122, which forms an additional input/output(I/O) bus in the computer system C. A read-only memory or ROM 124, whichcontains the instructions forming the BIOS and other fundamentaloperations, is connected to the X bus 122. A combo peripheral chip 126is also connected to the X bus 122. The combo peripheral chip 126includes a keyboard controller to receive keyboard and pointing deviceinputs from a user, an interface for connection to parallel and serialports 130 to provide for certain I/O capabilities, and a hard diskcontroller for interfacing to a hard disk drive 128. The comboperipheral chip 136 also includes a real-time clock (RTC) and basic andextended CMOS memory. The RTC provides various real-time clockfunctions, and the CMOS memory provides storage for various systemconfiguration information. Additionally, a floppy disk controller 132 isconnected to the X bus 122. The floppy disk controller 132 acts as aninterface between the X bus 122 and a floppy disk drive 134.

A power supply circuit 136 is also located in the computer system C. Theoutputs provided by the power supply circuit 136 include a 12-voltsupply signal +12 V, a 5-volt supply signal +5 V, and the supply signalPVDD, which varies between 3.3 volts and 5 volts depending on the typeof the CPU 101 used. The voltage detect sense pin VOLDET is provided tothe power supply circuit 136. The power supply signal PVDD is providedto the CPU 101, and in accordance with the present invention, isautomatically set at 3.3 volts or 5 volts depending on the state of thepin VOLDET.

Referring now to FIG. 2, regulator circuitry in the power supply circuit136 for generating the supply signal PVDD is shown. The regulatorcircuitry includes a power n-channel enhancement field effect transistor(FET) 200, whose drain is connected to the supply signal +5 V and whosesource is connected to the supply signal PVDD. A decoupling capacitor204 is connected between the drain of the power FET 200 and ground toremove high frequency noise. Decoupling and bulk capacitors 224 areconnected to the supply signal PVDD to maintain the voltage level atPVDD stable. The gate of the power FET 200 is connected to the outputpin of a voltage reference source 202. Any one of the standard voltagereference source chips generally available can be used, provided thatthe reference voltage is as described below, though the TL431 ispreferred. The gate of the power FET 200 is also coupled to the supplysignal +12 V through a resistor 203. The reference input of the voltagereference source 202 is connected to the collector of a bipolar junctiontransistor (BJT) 210, and the ground pin of the voltage reference source202 is connected to ground. The emitter of the BJT 210 is connected toground and its base is coupled to the voltage detect sense pin VOLDETthrough a resistor 212. A pullup resistor 214 is connected between thevoltage detect sense pin VOLDET and the supply signal +5 V. When a5-volt CPU 101 is placed into the socket 100, the sense pin VOLDET isleft floating. As a result, the pullup resistor 214 is able to pull thesense pin VOLDET to 5 volts. If a 3.3-volt CPU 101 is inserted into thesocket 100, then the sense pin VOLDET is driven low by the CPU 101.

A resistor 206 is connected between the source of the power FET 200 andthe collector of the BJT 210. Another resistor 208 is connected betweenthe collector of the BJT 210 and ground. The resistors 206 and 208 forma voltage divider circuit to provide a voltage at their common nodebased on the voltage of the supply signal PVDD. In addition, a capacitor216 is connected between the output pin and the reference pin of thereference voltage source 202. One side of a resistor 220 is alsoconnected to the output pin of the voltage reference source 202, and theother side of the resistor 220 is connected to one side of a capacitor218. The other side of the capacitor 218 is connected to the referencepin of the voltage reference source 202. The capacitors 216 and 218 andthe resistor 220 function to stabilize the voltage generated on theoutput pin of the voltage reference source 202. Any noise glitchoccurring on the reference pin of the voltage reference source 202 isfiltered by the combination of the resistor 220 and the capacitors 216and 218.

If the sense pin VOLDET is pulled high by the resistor 214, the BJT 210is turned on, thereby pulling the reference pin of the voltage referencesource 202 to approximately the collector-to-emitter saturation voltageof the BJT 210, which is approximately 0.2 to 0.3 volts. This turns thevoltage reference source 202 off, which allows the supply signal +12 Vto drive the gate of the power FET 200 to approximately 12 volts. As aresult, the voltage of the supply signal +5 V completely passes throughthe power FET 200 to drive the supply signal PVDD to 5 volts.

If a 3.3-volt CPU 101 is installed and the sense pin VOLDET is drivenlow, the BJT 210 is turned off. As a result, the voltage provided to thereference pin of the voltage reference source 202 is developed by thevoltage divider formed from the resistors 206 and 208. The voltage atthe reference pin of the voltage reference source 202 is thus dependentupon the power supply voltage PVDD. At steady state, the voltage at thereference pin of the voltage reference source 202 is driven to about 2.5volts. In response to this voltage, the voltage reference source 202drives the gate of the power FET 200 to approximately one thresholdvoltage (V_(T)) above 3.3 volts. The threshold voltage V_(T) of thepower FET 200 ranges from 3 to 10 volts. Consequently, due to thegate-to-source threshold voltage V.sub. of the power FET 200, thevoltage at the source of the power FET 200 is limited to approximately3.3 volts. A feedback path exists from the regulated supply signal PVDDthrough the resistor 206 to the reference pin of the voltage referencesource 202, which allows the supply signal PVDD to be maintained at arelatively stable 3.3 volts.

Optional zero ohm resistors 222 are inserted between the supply signal+5 V and the supply signal PVDD when the power FET 200 and all itsassociated circuitry are not installed. When optional resistors 222 areused instead of the power FET 200, the computer system C can be usedonly with 5-volt microprocessors.

Thus, a circuit has been described that automatically switches the powersupply voltage provided to a CPU, referred to as supply signal PVDD,between 3.3 volts and 5 volts. The circuit detects whether the CPUinstalled in a socket is a 3.3-volt part or a 5-volt part by determiningthe state of a voltage detect sense pin provided by the socket. If thevoltage detect sense pin is driven low, that indicates a 3.3-volt CPU isbeing used. If a 5-volt CPU is installed, the voltage detect sense pinis left floating by the CPU, which allows a pullup resistor to pull thevoltage detect sense pin high. The power supply voltage provided to theCPU is regulated through a power field effect transistor (FET). The gateof the power FET is connected to the output of a voltage referencesource and is coupled to a 12-volt supply signal. If the voltage detectsense pin is pulled high, the voltage reference source is turned off,allowing the 12-volt supply signal to drive the gate of the power FET.This in turn allows the power FET to pass a 5-volt supply signal to theCPU supply signal PVDD. If the voltage detect sense pin is pulled low,the voltage reference source is turned on to drive the gate of the powerFET to approximately one threshold voltage above 3.3 volts. In response,the power FET passes only 3.3 volts to the CPU supply signal PVDD.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the illustrated circuitry andconstruction and method of operation may be made without departing fromthe spirit of the invention.

We claim:
 1. A circuit for automatically switching a power supplyvoltage provided to a microprocessor used in a computer system between afirst voltage and a second voltage, said first voltage being greaterthan said second voltage, wherein the computer system includes a powersupply having a first power supply output providing the first voltage,and wherein the microprocessor provides a voltage detect sense signalpin for indicating whether the microprocessor requires the power supplyvoltage at the first voltage or the second voltage, the voltage detectsense signal being at a first state to indicate the first voltage, andthe voltage detect sense signal being at a second state to indicate thesecond voltage, the circuit comprising:a feedback circuit responsive tothe voltage detect sense signal and providing an output signal, saidoutput signal being driven to a third voltage if the voltage detectsense signal is at the first state, and said output signal being drivento a fourth voltage if the voltage detect sense signal is at the secondstate; and a field effect transistor having a gate driven by said outputsignal, a drain connected to the first power supply output, and a sourcecoupled to a microprocessor power supply voltage pin wherein the fieldeffect transistor drives the microprocessor power supply voltage to thefirst voltage if said output signal is at said third voltage and to thesecond voltage if said output signal is at said fourth voltage.
 2. Thecircuit of claim 1, wherein said field effect transistor has a thresholdvoltage, wherein said third voltage is approximately one thresholdvoltage greater than said first voltage, and wherein said fourth voltageis approximately one threshold voltage greater than said second voltage.3. The circuit of claim 2, wherein said first voltage is approximately 5volts and said second voltage is approximately 3.3 volts.
 4. The circuitof claim 2, wherein said feedback circuit includes:a voltage referencesource having an output pin and a reference pin, said output pinproviding said output signal; and a driver circuit responsive to saidvoltage detect sense signal and coupled to said reference pin fordriving said reference pin to a predetermined voltage if the voltagedetect sense signal is at the second state, wherein said reference pinbeing at said predetermined voltage causes said voltage reference sourceto drive said output signal to said fourth voltage.
 5. The circuit ofclaim 4, wherein the power supply includes a second computer systempower supply output providing a second power supply voltage, and whereinsaid feedback circuit further includes:a first resistor connectedbetween said output pin of said voltage reference source and said secondcomputer system power supply output, wherein said driver circuit drivessaid reference pin to turn off said voltage reference source if thevoltage detect sense signal is at the first state, and wherein saidsecond computer system power supply voltage drives said output signal tosaid third voltage through said first resistor if said voltage referencesource is turned off.
 6. The circuit of claim 5, wherein said drivercircuit includes:a second resistor connected between the microprocessorpower supply voltage pin and said reference pin; a third resistorconnected between said reference pin and a ground plane; and atransistor connected to said reference pin and coupled to the voltagedetect sense signal pin, said transistor driving said reference pin lowif the voltage detect sense signal is at the first state, saidtransistor being turned off if the voltage detect sense signal is at thesecond state, wherein said third resistor has a resistance in proportionto said second resistor to provide a voltage at said reference pin suchthat said output pin is driven to said fourth voltage when saidtransistor is turned off.
 7. The circuit of claim 6, wherein saidtransistor is a bipolar junction transistor having a collector connectedto said reference pin, a base coupled to said voltage detect sensesignal pin, and an emitter connected to said ground plane.
 8. Thecircuit of claim 7, wherein said first voltage is approximately 5 voltsand said second voltage is approximately 3.3 volts.
 9. The circuit ofclaim 1, wherein the microprocessor drives the voltage detect sensesignal low to indicate that the microprocessor requires themicroprocessor power supply signal at the second voltage, and whereinthe microprocessor leaves the voltage detect sense signal undriven toindicate that the microprocessor requires the microprocessor powersupply signal at the first voltage, the circuit further comprising:apullup resistor connected to the voltage detect sense signal pin forpulling the voltage detect sense signal high if the voltage detect sensesignal is undriven by the microprocessor.
 10. A computer system,comprising:a socket having a voltage detect sense pin and a power supplypin; a microprocessor having a plurality of pins for insertion into saidsocket, one of said microprocessor pins being connected to said voltagedetect sense pin and-another of said microprocessor pins being connectedto said power supply pin, wherein said microprocessor drives saidvoltage detect sense pin to a first state if said microprocessorrequires said power supply pin to be driven to a first voltage, andwherein said microprocessor drives said voltage detect sense pin to asecond state if said microprocessor requires said power supply pin to bedriven to a second voltage, said first voltage being greater than saidsecond voltage; a power supply having a first power supply output forproviding said first voltage; and a circuit coupled to said power supplyand said socket for automatically switching said power supply pinbetween said first voltage and said second voltage, including:a feedbackcircuit coupled to said voltage detect sense pin for providing an outputsignal, said output signal being driven to a third voltage if saidvoltage detect sense pin is at said first state, and said output signalbeing driven to a fourth voltage if said voltage detect sense pin is atsaid second state; and a field effect transistor having a gate driven bysaid output signal, a drain connected to said first power supply output,and a source coupled to said power supply pin, wherein said field effecttransistor drives said power supply pin to said first voltage if saidoutput signal is at said third voltage and to said second voltage ifsaid output signal is at said fourth voltage.
 11. The computer system ofclaim 10, wherein said field effect transistor has a threshold voltage,wherein said third voltage is approximately one threshold voltagegreater than said first voltage, and wherein said fourth voltage isapproximately one threshold voltage greater than said second voltage.12. The computer system of claim 11, wherein said first voltage isapproximately 5 volts and said second voltage is approximately 3.3volts.
 13. The computer system of claim 11, wherein said feedbackcircuit includes:a voltage reference source having an output pin and areference pin, said output pin providing said output signal; and adriver circuit coupled to said voltage detect sense pin and coupled tosaid reference pin for driving said reference pin to a predeterminedvoltage if said voltage detect sense pin is at said second state,wherein said reference pin being at said predetermined voltage causessaid voltage reference source to drive said output signal to said fourthvoltage.
 14. The computer system of claim 13, wherein said power supplyprovides a second computer system power supply output providing a secondpower supply voltage, and wherein said feedback circuit furtherincludes:a first resistor connected between said output pin of saidvoltage reference source and said second computer system power supplyoutput, wherein said driver circuit drives said reference pin to turnoff said voltage reference source if the voltage detect sense signal isat said first state, and wherein said second computer system powersupply voltage drives said output signal to said third voltage throughsaid first resistor if said voltage reference source is turned off. 15.The computer system of claim 14, wherein said driver circuit includes:asecond resistor connected between said source of said field effecttransistor and said reference pin; a third resistor connected betweensaid reference pin and a ground and a transistor connected to saidreference pin and coupled to said voltage detect sense pin, saidtransistor driving said reference pin low if said voltage detect sensepin is at said first state, said transistor being turned off if saidvoltage detect sense pin is at said second state, wherein said thirdresistor has a resistance in proportion to said second resistor toprovide a voltage at said reference pin such that said output pin isdriven to said fourth voltage when said transistor is turned off. 16.The computer system of claim 15, wherein said transistor is a bipolarjunction transistor having a collector connected to said reference pin,a base coupled to said voltage detect sense pin, and an emitterconnected to said ground plane.
 17. The computer system of claim 16,wherein said first voltage is approximately 5 volts and said secondvoltage is approximately 3.3 volts.
 18. The computer system of claim 10,wherein said microprocessor drives said voltage detect sense pin low toindicate that said microprocessor requires said power supply pin to bedriven at said second voltage, and wherein said microprocessor leavessaid voltage detect sense pin undriven to indicate that saidmicroprocessor requires said power supply pin to be driven at said firstvoltage, said circuit further comprising:a pullup resistor connected tosaid voltage detect sense pin for pulling said voltage detect sense pinhigh if said voltage detect sense pin is undriven by saidmicroprocessor.